Basically you need to do the below steps:
1. From CRXDE: navigate to /libs/foundation/components/text/dialog/items/tab1/items/text/rtePlugins/
2. Create a new node as below:
3. Add a property to the newly created 'format' node as below:
4. Save your work and exit out of CRXDE.
Now when you add a Text component to your page, the 'Underline' functionality should be disabled.
Hope this helps.